Medical device

ABSTRACT

A medical device is described having a handle and an end effector coupled to the handle. The end effector has at least one electrode for providing electrical signals to a tissue or vessel to be treated. An RF drive circuit is provided for generating an RF drive signal that is applied to the end effector electrode. The RF drive circuit includes a resonant circuit and a frequency controller is used to vary the frequency of a signal passed through the resonant circuit in order to control the power supplied to the end effector electrode.

The present invention relates to the field of medical devices and inparticular, although not exclusively, to medical cauterization andcutting devices. The invention also relates to drive circuits andmethods for driving such medical devices.

Many surgical procedures require cutting or ligating blood vessels orother internal tissue. Many surgical procedures are performed usingminimally invasive techniques, a hand-held cauterization device is usedby the surgeon to perform the cutting or ligating. The existinghand-held cauterization devices require a desk top power supply andcontrol electronics that are connected to the device through anelectrical supply line. FIG. 10 illustrates such an existing hand-heldcauterization device currently in use.

It has been known for a number of years that these existing devices arecumbersome and difficult to use during a surgical operation due to thelarge size of the supply and control electronics and due to thetethering of the hand-held cauterization device to the supply andcontrol electronics. It has also been known for a number of years thatthese problems would be overcome by providing a battery poweredhand-held cauterization device in which the power and controlelectronics are mounted within the device itself, such as within thehandle of the device. However, it is not a simple matter ofminiaturising the electronics. The power that has to be supplied to thedevice during the surgical procedure and the current design of theelectronics is such that large capacitors, inductors and transformers aswell as heat sinks and fans are required. FIG. 11 illustrates in moredetail the different parts of the supply and control electronics thatare used in the existing design. Whilst it is possible to reduce thesize of the sensing and control electronics, other parts of thecircuitry cannot be miniaturised in this way.

In particular, the existing electronics design uses circuitry forproviding an adjustable 24 Volt power supply; FETs and associated drivecircuitry; a transformer for increasing the supply voltage; andfiltering circuitry to remove harmonics from the square wave voltagelevels that are generated by the FET switches and the transformer. Giventhe voltage levels and the power levels used to drive the cauterizationdevice, the transformers and output filters all have to be relativelybulky devices and large heat sinks and a fan are required to cool theFET switches.

The present invention aims to provide an alternative circuit design thatwill allow the miniaturisation of the circuitry so that it can be builtinto the hand-held cauterization device, whilst still being able toprovide the power and control required for the medical procedure.

The present invention provides a medical device comprising an endeffector having at least one electrical contact a radio frequency, RF,generation circuit for generating an RF drive signal and to provide theRF drive signal to the at least one electrical contact and wherein theRF generation circuit comprises a resonant circuit. In one embodiment,the radio frequency generation circuit comprises switching circuitrythat generates a cyclically varying signal, such as a square wavesignal, from a DC supply and the resonant circuit is configured toreceive the cyclically varying signal from the switching circuitry. TheDC supply is preferably provided by one or more batteries that can bemounted in a housing (such as a handle) of the device.

According to another aspect, the invention provides a medical devicecomprising: a handle for gripping by a user; an end effector coupled tothe handle, the end effector having at least one electrical contact;battery terminals for connecting to one or more batteries; a radiofrequency, RF, generation circuit coupled to said battery terminals andoperable to generate an RF drive signal and to provide the RF drivesignal to the at least one electrical contact of said end effector;wherein the frequency generation circuit comprises: switching circuitryfor generating a cyclically varying signal (which may be a square wavepulse width modulated signal) from a potential difference across saidbattery terminals; and a resonant drive circuit coupled to saidswitching circuitry and operable to filter the cyclically varying signalgenerated by the switching circuitry; and wherein the RF drive signal isobtained using an output signal from said resonant circuit.

The medical device may also comprise a control circuit (which maycomprise hardware and/or software) that varies the frequency of the RFdrive signal. The control circuit may vary the frequency based on ameasurement of the RF drive signal in order to control at least one ofthe power, voltage and/or current delivered to the at least oneelectrical contact of the end effector. In a preferred embodiment, themeasurement is obtained from a sampling circuit that operatessynchronously with respect to the frequency of the RF drive signal. Thefrequency at which the sampling circuit samples the sensed signal may bean integer fraction of the frequency of the RF drive signal.

In one embodiment, the control circuit varies the frequency of the RFdrive signal around (preferably just above or just below) the resonantfrequency of the resonant circuit. The resonant characteristic of theresonant circuit may vary with a load connected to the at least oneelectrical contact and the control circuit may be arranged to vary theRF drive frequency to track changes in the resonant characteristic ofthe resonant circuit.

According to another aspect, the invention provides a medical devicecomprising: a handle for gripping by a user; an end effector coupled tothe handle and having at least one electrical contact; a radiofrequency, RF, generation circuit operable to generate an RF drivesignal and to provide the RF drive signal to the at least one electricalcontact; and a control circuit operable to vary the frequency of the RFdrive signal to control at least one of the power, the voltage and thecurrent provided to the at least one contact of the end effector.

The RF generation circuit may comprise a signal generator that generatesa cyclically varying signal at the RF frequency; and a frequencydependent attenuator that attenuates the cyclically varying signal independence upon the frequency of the cyclically varying signal. Thefrequency dependent attenuator may be a lossless attenuator and maycomprise a resonant circuit having a resonant frequency at or near theRF frequency of the cyclically varying signal.

The present invention also provides a medical device comprising: ahandle for gripping by a user; an end effector coupled to the handle andhaving at least one electrical contact; a radio frequency, RF,generation circuit operable to generate an RF drive signal and toprovide the RF drive signal to the at least one electrical contact; aninput for receiving a sensed signal that varies with the RF drive signalapplied to the at least one electrical contact; a sampling circuit forsampling the sensed signal received at said input; a measurement circuitoperable to make measurements of the RF drive signal using samplesobtained from the sampling circuit; and a control circuit operable tocontrol the RF generation circuit in dependence upon the measurementsmade by the measurement circuit, to vary the frequency of the generatedRF drive signal; wherein the sampling circuit is operable to sample thesensed signal at a sampling frequency that varies in synchronism withthe frequency of the RF drive signal.

The invention also provides a method of operating a medical devicecomprising generating an RF signal and applying the RF signal to atleast one electrode of an end effector of the medical device andcontrolling the frequency of the generated RF signal to control at leastone of the power, current, and voltage applied to the at least oneelectrode.

According to another aspect, the invention provides a method ofcauterising a vessel or tissue, the method comprising: gripping thevessel or tissue with an end effector of a medical device; applying anRF signal to at least one electrode of the end effector that is incontact with the vessel or tissue; and controlling the frequency of theRF signal to control at least one of the power, current, and voltageapplied to the tissue to perform the cauterisation.

The above methods may use the above described medical device, althoughthat is not essential.

The controlling step may vary the frequency of the RF signal to controlthe power applied to the tissue or vessel, and the method may furthercomprise obtaining measurements of the impedance of the tissue or vesseland varying the desired power applied to the tissue or vessel independence upon the obtained impedance measurements.

These and various other features and aspects of the invention willbecome apparent from the following detailed description of embodimentswhich are described with reference to the accompanying Figures in which:

FIG. 1 illustrates a hand-held cauterization device that has batteriesand drive and control circuitry mounted into a handle portion of thedevice;

FIG. 2 is a part block part schematic diagram illustrating the maincomponents of the RF drive circuitry and control circuitry used in oneembodiment of the invention;

FIG. 3 is a block diagram illustrating the main components of acontroller used to control the operation of the RF drive circuitryillustrated in FIG. 2;

FIG. 4 is a timing diagram illustrating the RF drive signals applied tothe cauterization device and illustrating a way in which synchronoussamples may be obtained to measure the drive signals;

FIG. 5a is a plot illustrating limits that are placed on voltage andcurrent supplied to the cauterization device illustrated in FIG. 1;

FIG. 5b illustrates a resulting power plot obtained by combining thecurrent and voltage plots illustrated in FIG. 5 a;

FIG. 6 is a plot illustrating the way in which the resonantcharacteristics of the RF drive circuit illustrated in FIG. 2 varieswith different loads;

FIG. 7 is a flow chart illustrating the operation of a frequency controlalgorithm used to control the frequency of the RF drive signals appliedto the cauterization device;

FIG. 8 is a plot illustrating one way in which the power limit can bevaried by the control electronics during a surgical procedure;

FIG. 9 is a part block part schematic diagram illustrating the maincomponents of another RF drive circuit and control circuit embodying theinvention;

FIG. 10 illustrates the form of a prior art hand-held cauterizationdevice which is connected to power supply and control electronics via apower supply line; and

FIG. 11 is a plan view illustrating the different components of theexisting electronics used to drive and control the hand-heldcauterization device illustrated in FIG. 10.

MEDICAL DEVICE

Many surgical procedures require cutting or ligating blood vessels orother vascular tissue. With minimally invasive surgery, surgeons performsurgical operations through a small incision in the patient's body. As aresult of the limited space, surgeons often have difficulty controllingbleeding by clamping and/or tying-off transected blood vessels. Byutilizing electrosurgical forceps, a surgeon can cauterize,coagulate/desiccate, and/or simply reduce or slow bleeding bycontrolling the electrosurgical energy applied through jaw members ofthe electrosurgical forceps.

FIG. 1 illustrates the form of an electrosurgical medical device 1 thatis designed for minimally invasive medical procedures, according to oneembodiment of the present invention. As shown, the device 1 is a selfcontained device, having an elongate shaft 3 that has a handle 5connected to the proximal end of the shaft 3 and an end effector 7connected to the distal end of the shaft 3. In this embodiment, the endeffector 7 comprises medical forceps 9 and a cutting blade (not shown)that are controlled by the user manipulating control levers 11 and 13 ofthe handle 5.

During a surgical procedure, the shaft 3 is inserted through a trocar togain access to the patient's interior and the operating site. Thesurgeon will manipulate the forceps 9 using the handle 5 and the controllevers 11 and 13 until the forceps 9 are located around the vessel to becauterised. Electrical energy at an RF frequency (it has been found thatfrequencies above about 50 kHz do not affect the human nervous system)is then applied, in a controlled manner, to the forceps 9 to perform thedesired cauterisation. As shown in FIG. 1, in this embodiment, thehandle 5 houses batteries 15 and control electronics 17 for generatingand controlling the electrical energy required to perform thecauterisation. In this way, the device 1 is self contained in the sensethat it does not need a separate control box and supply wire to providethe electrical energy to the forceps 9.

RF Drive Circuitry

FIG. 2 is a part schematic part block diagram illustrating the RF driveand control circuitry 20 used in this embodiment to generate and controlthe RF electrical energy supplied to the forceps 9. As will be explainedin more detail below, in this embodiment, the drive circuitry 20 is aresonant based circuit and the control circuitry operates to control theoperating frequency of the drive signal so that it is varied around theresonant frequency of the drive circuit, which in turn controls theamount of power supplied to the forceps 9. The way that this is achievedwill become apparent from the following description.

As shown in FIG. 2, the drive circuitry 20 comprises the above describedbatteries 15 that are arranged to supply, in this example, 0V and 24Vrails. An input capacitor (C_(in)) 21 is connected between the 0V andthe 24V rails for providing a low source impedance. A pair of FETswitches 23-1 and 23-2 (both of which are N-channel in this embodimentto reduce power losses) is connected in series between the 0V rail andthe 24V rail. FET gate drive circuitry 25 is provided that generates twodrive signals—one for driving each of the two FETs 23. The FET gatedrive circuitry 25 generates drive signals that causes the upper FET(23-1) to be on when the lower FET (23-2) is off and vice versa. Thiscauses the node 27 to be alternately connected to the 24V rail (when FET23-1 is switched on) and the 0V rail (when the FET 23-2 is switched on).FIG. 2 also shows the internal parasitic diodes 28-1 and 28-2 of thecorresponding FETs 23, which conduct during any periods that the FETs 23are open.

As shown in FIG. 2, the node 27 is connected to acapacitor-inductor-inductor resonant circuit 28 formed by capacitorC_(s) 29, inductor L_(s) 31 and inductor L_(m) 33. The FET gate drivingcircuitry 25 is arranged to generate drive signals at a drive frequency(f_(d)) that opens and closes the FET switches 23 at around the resonantfrequency of the resonant circuit 28. As a result of the resonantcharacteristic of the resonant circuit 28, the square wave voltage atnode 27 will cause a substantially sinusoidal current at the drivefrequency (f_(d)) to flow within the resonant circuit 28. As illustratedin FIG. 2, the inductor L_(m) 33 is the primary of a transformer 35, thesecondary of which is formed by inductor L_(sec) 37. The transformer 35up-converts the drive voltage (V_(d)) across inductor L_(m) 33 to theload voltage (V_(L)) that is applied to the load (represented by theload resistance R_(load) 39 in FIG. 2) corresponding to the impedance ofthe forceps' jaws and any tissue or vessel gripped by the forceps 9. Asshown in FIG. 2, a pair of DC blocking capacitors C_(bl) 40-1 and 40-2is provided to prevent any DC signal being applied to the load 39.

In this embodiment, the amount of electrical power supplied to theforceps 9 is controlled by varying the frequency of the switchingsignals used to switch the FETs 23. This works because the resonantcircuit 28 acts as a frequency dependent (lossless) attenuator. Thecloser the drive signal is to the resonant frequency of the resonantcircuit 28, the less the drive signal is attenuated. Similarly, as thefrequency of the drive signal is moved away from the resonant frequencyof the circuit 28, the more the drive signal is attenuated and so thepower supplied to the load reduces. In this embodiment, the frequency ofthe switching signals generated by the FET gate drive circuitry 25 iscontrolled by a controller 41 based on a desired power to be deliveredto the load 39 and measurements of the load voltage (V_(L)) and of theload current (i_(L)) obtained by conventional voltage sensing circuitry43 and current sensing circuitry 45. The way that the controller 41operates will be described in more detail below.

Controller

FIG. 3 is a block diagram illustrating the main components of thecontroller 41. In this embodiment, the controller 41 is amicro-processor based controller and so most of the componentsillustrated in FIG. 3 are software based components. However, a hardwarebased controller 41 may be used instead. As shown, the controller 41includes synchronous I,Q sampling circuitry 51 that receives the sensedvoltage and current signals from the sensing circuitry 43 and 45 andobtains corresponding samples which are passed to a power, V_(rms) andI_(rms) calculation module 53. The calculation module 53 uses thereceived samples to calculate the RMS voltage and RMS current applied tothe load 39 (forceps 9 and tissue/vessel gripped thereby) and from themthe power that is presently being supplied to the load 39. Thedetermined values are then passed to a frequency control module 55 and amedical device control module 57. The medical device control module 57uses the values to determine the present impedance of the load 39 andbased on this determined impedance and a pre-defined algorithm,determines what set point power (P_(set)) should be applied to thefrequency control module 55. The medical device control module 57 is inturn controlled by signals received from a user input module 59 thatreceives inputs from the user (for example pressing buttons oractivating the control levers 11 or 13 on the handle 5) and alsocontrols output devices (lights, a display, speaker or the like) on thehandle 5 via a user output module 61.

The frequency control module 55 uses the values obtained from thecalculation module 53 and the power set point (P_(set)) obtained fromthe medical device control module 57 and predefined system limits (to beexplained below), to determine whether or not to increase or decreasethe applied frequency. The result of this decision is then passed to asquare wave generation module 63 which, in this embodiment, incrementsor decrements the frequency of a square wave signal that it generates by1 kHz, depending on the received decision. As those skilled in the artwill appreciate, in an alternative embodiment, the frequency controlmodule 55 may determine not only whether to increase or decrease thefrequency, but also the amount of frequency change required. In thiscase, the square wave generation module 63 would generate thecorresponding square wave signal with the desired frequency shift. Inthis embodiment, the square wave signal generated by the square wavegeneration module 63 is output to the FET gate drive circuitry 25, whichamplifies the signal and then applies it to the FET 23-1. The FET gatedrive circuitry 25 also inverts the signal applied to the FET 23-1 andapplies the inverted signal to the FET 23-2.

Drive Signals and Signal Measurements

FIG. 4 is a signal plot illustrating the switching signals applied tothe FETs 23; a sinusoidal signal representing the measured current orvoltage applied to the load 39; and the timings when the synchronoussampling circuitry 51 samples the sensed load voltage and load current.In particular, FIG. 4 shows the switching signal (labelled PWM1H)applied to upper FET 23-1 and the switching signal (labelled PWM1L)applied to lower FET 23-2. Although not illustrated for simplicity,there is a dead time between PWM1H and PWM1L to ensure that that bothFETs 23 are not on at the same time. FIG. 4 also shows the measured loadvoltage/current (labelled OUTPUT). Both the load voltage and the loadcurrent will be a sinusoidal waveform, although they may be out ofphase, depending on the impedance of the load 39. As shown, the loadcurrent and load voltage are at the same drive frequency (f_(d)) as theswitching signals (PWM1H and PWM1L) used to switch the FETs 23.Normally, when sampling a sinusoidal signal, it is necessary to samplethe signal at a rate corresponding to at least twice the frequency ofthe signal being sampled—i.e. two samples per period. However, as thecontroller 41 knows the frequency of the switching signals, thesynchronous sampling circuit 51 can sample the measured voltage/currentsignal at a lower rate. In this embodiment, the synchronous samplingcircuit 51 samples the measured signal once per period, but at differentphases in adjacent periods. In FIG. 4, this is illustrated by the “I”sample and the “Q” sample. The timing that the synchronous samplingcircuit 51 makes these samples is controlled, in this embodiment, by thetwo control signals PWM2 and PWM3, which have a fixed phase relative tothe switching signals (PWM1H and PWM1L) and are out of phase with eachother (preferably by quarter of the period as this makes the subsequentcalculations easier). As shown, the synchronous sampling circuit 51obtains an “I” sample on every other rising edge of the PWM2 signal andthe synchronous sampling circuit 51 obtains a “Q” sample on every otherrising edge of the PWM3 signal. The synchronous sampling circuit 51generates the PWM2 and PWM3 control signals from the square wave signaloutput by the square wave generator 63 (which is at the same frequencyas the switching signals PWM1H and PWM1L). Thus when the frequency ofthe switching signals is changed, the frequency of the sampling controlsignals PWM2 and PWM3 also changes (whilst their relative phases staythe same). In this way, the sampling circuitry 51 continuously changesthe timing at which it samples the sensed voltage and current signals asthe frequency of the drive signal is changed so that the samples arealways taken at the same time points within the period of the drivesignal. Therefore, the sampling circuit 51 is performing a “synchronous”sampling operation instead of a more conventional sampling operationthat just samples the input signal at a fixed sampling rate defined by afixed sampling clock.

The samples obtained by the synchronous sampling circuitry 51 are thenpassed to the power, V_(rms) and I_(rms) calculation module 53 which candetermine the magnitude and phase of the measured signal from just one“I” sample and one “Q” sample of the load current and load voltage.However, in this embodiment, to achieve some averaging, the calculationmodule 53 averages consecutive “I” samples to provide an average “I”value and consecutive “Q” samples to provide an average “Q” value; andthen uses the average I and Q values to determine the magnitude andphase of the measured signal (in a conventional manner). As thoseskilled in the art will appreciate, with a drive frequency of about 400kHz and sampling once per period means that the synchronous samplingcircuit 51 will have a sampling rate of 400 kHz and the calculationmodule 53 will produce a voltage measure and a current measure every0.01 ms. The operation of the synchronous sampling circuit 51 offers animprovement over existing products, where measurements can not be madeat the same rate and where only magnitude information is available (thephase information being lost).

Limits

As with any system, there are certain limits that can be placed on thepower, current and voltage that can be delivered to the forceps 9. Thelimits used in this embodiment and how they are controlled will now bedescribed.

In this embodiment, the RF drive circuitry 20 is designed to deliver apower limited sine wave into tissue with the following requirements:

1) Supplied with a nominally 24V DC supply2) Substantially sinusoidal output waveform at approximately 400 kHz3) Power limited output of 45 W4) Current limited to 1.4 A_(rms) and voltage limited to 85V_(rms)

The last two requirements are represented graphically in FIGS. 5a and 5b. In particular, FIG. 5a illustrates idealised plots of voltage andcurrent for loads between 1 Ohm and 10 k Ohms on a logarithmic scale;and FIG. 5b illustrates the power delivered to the load 39 for loadsbetween 1 Ohm and 10 k Ohms.

The frequency control module 55 maintains data defining these limits anduses them to control the decision about whether to increase or decreasethe excitation frequency.

Resonant Characteristic and Frequency Control

As mentioned above, the amount of electrical power supplied to theforceps 9 is controlled by varying the frequency of the switchingsignals used to switch the FETs 23. This is achieved by utilising thefact that the impedance of the resonant circuit 28 changes rapidly withfrequency. Therefore by changing the frequency of the switching signals,the magnitude of the current through the resonant circuit 28, and hencethrough the load 39, can be varied as required to regulate the outputpower.

As those skilled in the art will appreciate, the resonant circuit 28 iscoupled to a load 39 whose impedance will vary during the surgicalprocedure. Indeed the medical device control module 57 uses thisvariation to determine whether the tissue or vessel has been cauterised,coagulated/desiccated. The varying impedance of the load 39 changes thefrequency characteristic of the RF drive circuit 20 and hence thecurrent that flows through the resonant circuit 28. This is illustratedin FIG. 6, which is a plot 65 illustrating the way in which the currentthrough the resonant circuit 28 varies with the drive frequency for afixed value of load impedance. As the impedance of the load 39increases, the resonant characteristic 65 will change shape (the peakmay grow or reduce in height) and will move to the left and as theimpedance of the load decreases it will change its shape and move to theright. Therefore, the frequency control module 55 must operate quicklyenough to track the changes in the resonant characteristic 65. This iseasily achievable in this embodiment, where power, current and voltagemeasurements are available every 0.01 ms. In general terms, measurementswould only be required at a rate of about once every 0.1 s to track thechanges. However sudden changes in the resonant characteristic 65 canoccur, which the frequency control module 55 cannot track. When thishappens, the frequency control module 55 resets the operating frequencyto a value where it knows that it will be on one side of thecharacteristic.

As the impedance of the resonant circuit 28 increases sharply both aboveand below resonance, it is possible to operate the RF drive circuit 20either above or below the resonant frequency. In this embodiment, thefrequency control module 55 controls the operation of the drive circuit20 so that it operates slightly above the resonant frequency as thisshould lead to lower switching losses through the FETs 23.

FIG. 7 illustrates the processing performed in this embodiment by thecalculation module 53 and the frequency control module 55. As shown, atthe beginning of the process in step s1, the control module 55 turns onthe RF drive signal at the system defined maximum frequency by passingan initialisation signal to the square wave generation module 63.Provided the control module 55 has not received, in step s3, a powerdown signal from the medical device control module 57, the processingproceeds to step s5 where the calculation module 53 obtains the voltageand current samples from the synchronous sampling circuitry 51. In steps7 the calculation module 53 calculates the square of the voltage andthe square of the current and the delivered power by multiplying themeasured voltage by the measured current. These calculated values arethen passed to the frequency control module 55 which compares, in steps9, the values with the defined limits for the applied voltage, currentand power. The voltage and current limits are static limits that aredefined in advance. However, the power limit depends on the medicalprocedure and is defined by the power set point (P_(set)) provided bythe medical device control module 57. If each of the measured values isbelow the corresponding limit then, in step s11, the frequency controlmodule 55 decides to decrease the drive frequency and a decrease commandis passed to the square wave generator 63. At the start of theprocessing, the drive frequency is set to a defined maximum value (inthis embodiment 500 kHz), which will always be above the resonant peakof the characteristic 65, regardless of the load impedance. Therefore,regardless of the load 39, the initial operating frequency should be onthe right hand side of the resonant plot shown in FIG. 6. By decreasingthe drive frequency, the drive frequency will get closer to the resonantfrequency of the resonant circuit 28. As a result, the applied currentwill increase and more power will be delivered to the load 39. Theprocessing then returns to step s3 and the above process is repeated.

Therefore, the current and power applied to the load 39 should increaseuntil one of the limits is reached. At this point, the control module 55will determine, in step s9, that a limit has been reached and so willproceed to step s13, where the control module 55 decides to increase thedrive frequency and sends the square wave generation module 63 anincrease command. This will cause the drive frequency to move away fromthe resonant frequency of the circuit 28 and so the current and powerdelivered to the load 39 will reduce. The processing will then return tostep s3 as before.

Thus, by starting on one side of the resonant peak and slowly moving thedrive frequency towards and away from the resonant peak, the current andpower level applied to the load 39 can be controlled within the definedlimits even as the impedance of the load changes and the resonantcharacteristic 65 of the resonant circuit 28 changes as thetissue/vessel is cauterised.

As those skilled in the art will appreciate, it would also be possibleto start on the left hand side of the resonant peak and increase thedrive frequency to increase the delivered power and decrease the drivefrequency to decrease the delivered power.

Medical Device Control Module

As mentioned above, the medical device control module 57 controls thegeneral operation of the cauterisation device 1. It receives user inputsvia the user input module 59. These inputs may specify that the jaws ofthe forceps 9 are now gripping a vessel or tissue and that the userwishes to begin cauterisation. In response, in this embodiment, themedical device control module 57 initiates a cauterisation controlprocedure. Initially, the medical device control module 57 sends aninitiation signal to the frequency control module 55 and obtains currentand power measurements from the calculation module 53. The medicaldevice control module 57 then checks the obtained values to make surethat the load 39 is not open circuit or short circuit. If it is not,then the medical device control module 57 starts to vary the power setpoint to perform the desired cauterisation. FIG. 8 is a plotillustrating the way in which the medical device control module 57 mayvary the set point power to achieve the desired cauterisation procedure.Various other techniques and other power delivery algorithms may also beused.

As shown in FIG. 8, during an initial period 71 the medical devicecontrol module 57 pulses the set point power between zero and about 10Watts. Then during a main cauterisation period 73 (which typically lastsfor about 5 seconds) the medical device control module 57 pulses the setpoint power between zero and 50 Watts. During this period, the medicalcontrol device receives the power and voltage measurements from thecalculation module 53 and calculates from them the impedance of the load39. The medical device control module 57 determines that thecauterisation is complete when the calculated impedance exceeds athreshold. Finally, the medical device control module 57 performs aterminating procedure during a terminating period 75. During theterminating procedure, the medical device control module 57 varies theset point power and checks that cauterisation has been achieved (bychecking the Impedance of the load using the measured power and currentvalues) and re-enters the main cauterisation period again if itdetermines that cauterisation has not been completed.

Resonant Circuit Design

The way that the values of the inductors and capacitors were chosen inthis embodiment will now be described. As those skilled in the art willappreciate, other design methodologies may be used.

The complex impedance of the circuit shown in FIG. 2 can be approximatedby the following equation:

$\begin{matrix}{Z = {{j\; 2\; \pi \; {fL}_{s}} + \frac{1}{j\; 2\; \pi \; {fC}_{s}} + \frac{j\; 2\; \pi \; {fL}_{m}R_{load\_ ref}}{{j\; 2\; \pi \; {fL}_{m}} + R_{load\_ ref}} + {R_{s}.}}} & (1)\end{matrix}$

Where:

R_(load) _(_) _(ref) is the load resistance referred to the primary (bythe square of the turns ratio);R_(s) represents the equivalent series resistance of the inductor,transformer capacitor and switching devices.

All other component non-idealities are ignored and the transformer isconsidered to be ideal as a first approximation.

Assuming that R_(s) is small, when the load is open circuit (ie R_(load)_(_) _(ref) is infinite) the resonant frequency can be shown to be:

$\begin{matrix}{f_{\min} = \frac{1}{2\; \pi \sqrt{\left( {L_{s} + L_{m}} \right)C_{s}}}} & (2)\end{matrix}$

Similarly, when the load is short circuit (ie R_(load) _(_) _(ref) iszero) the resonant frequency can be shown to be:

$\begin{matrix}{f_{\max} = \frac{1}{2\; \pi \sqrt{L_{s}C_{s}}}} & (3)\end{matrix}$

Assuming R_(s) is small: at each frequency between f_(min) and f_(max)there is a value of the load, R_(load), at which the greatest power canbe dissipated in the load. This maximum power can be shown to be largeat frequencies near f_(min) and f_(max), and has a minimum at thecritical frequency, fc. We refer to this power as P_(max) _(_) _(fc).Starting with (1) it can be shown that the following relationship holds:

$\begin{matrix}{L_{m} = \frac{2V_{s}^{2}}{2\; \pi \; f_{c}P_{max\_ fc}}} & (4)\end{matrix}$

where V_(s) is the supply voltage.

It can be shown that the load at which equation (4) holds is given by:

R _(load) _(_) _(ref)=2πfL _(m)  (5)

Furthermore from (1) a relationship between f_(min), f_(c) and f_(max)can be established:

$\begin{matrix}{f_{\min} = \sqrt{\left( {2\; \pi \; f_{c}} \right)^{2}\frac{\left( {2\; \pi \; f_{\max}} \right)^{2} + \left( {2\; \pi \; f_{c}} \right)^{2}}{{3\left( {2\; \pi \; f_{\max}} \right)^{2}} - \left( {2\; \pi \; f_{c}} \right)^{2}}}} & (6)\end{matrix}$

From (6) it can be shown that f_(min)<f_(c)<f_(max). If the circuit isto operate at f_(c), then equation (4) gives an upper bound on theworst-case power delivered across a range of loads.

From (1), it can be shown that the efficiency of the circuit atresonance may be written as:

$\begin{matrix}{\eta = {\frac{\left( \frac{\left( {2\; \pi \; f_{c}} \right)^{2}L_{m}^{2}R_{load}}{{\left( {2\; \pi \; f_{c}} \right)^{2}L_{m}^{2}} + R_{load}^{2}} \right)}{{Re}(Z)} = \frac{\left( {2\; \pi \; f_{c}} \right)^{2}L_{m}^{2}R_{load\_ ref}}{{R_{s}\left( {{\left( {2\; \pi \; f_{c}} \right)^{2}L_{m}^{2}} + R_{load\_ ref}^{2}} \right)} + {\left( {2\; \pi \; f_{c}} \right)^{2}L_{m}^{2}R_{load\_ ref}}}}} & (7)\end{matrix}$

From (7) it may be shown that the efficiency is a maximum when R_(load)_(_) _(ref)=2πfL_(m), i.e. when (5) holds. Therefore the system isdesigned to operate around the point of maximum efficiency.

Design Procedure

For this specific embodiment of the design the following parameters werechosen:

-   -   Battery voltage of 24V however battery voltage droops with        discharge and load so V_(s) _(_) _(sq)=18V (square wave peak to        peak voltage) was used    -   R_(load)=45 W (maximum power into the load)    -   V_(load)=85 Vrms (maximum voltage into the load)    -   I_(load)=1.4 Arms (maximum current into the load)    -   f_(c)=430 kHz (centre or critical switching frequency)    -   f_(max)=500 kHz (maximum switching frequency, which is the upper        resonant frequency)    -   f_(min)=380 kHz (approximate minimum switching frequency—needs        to be calculated)

Given these values, f_(min) can be computed using (6):

$f_{\min} = \sqrt{\left( {2\; \pi \; 430\; k} \right)^{2}\frac{\left( {2\; \pi \; 500\; k} \right)^{2} + \left( {2\; \pi \; 430\; k} \right)^{2}}{{3\left( {2\; \pi \; 500\; k} \right)^{2}} - \left( {2\; \pi \; 430\; k} \right)^{2}}}$f_(min) = 377  kHz

Resonant circuits produce sinusoidal waveforms therefore the inputsquare wave voltage (V_(s) _(_) _(sq)) needs to be converted into theRMS of the fundamental switching frequency (V_(s)).

$\begin{matrix}{V_{s} = {\frac{4}{\pi}\frac{Vs\_ sq}{2\sqrt{2}}}} \\{= {\frac{4}{\pi}\frac{18\mspace{14mu} V}{2\sqrt{2}}}} \\{= {8.1\mspace{14mu} V_{rms}}}\end{matrix}$

The power into the load (P_(load)) is set by L_(m). Using (4) thetransformer magnetising inductance (L_(m)) can be determined. Thisensures that at the critical frequency, f_(c), the required power isdelivered:

$\begin{matrix}{L_{m} = \frac{2\; V_{s}^{2}}{2\; \pi \; f_{c}P_{load}}} \\{= \frac{2 \times 8.1\mspace{14mu} V_{rms}^{2}}{2\; \pi \; \times 420\mspace{14mu} {kHz} \times 45\mspace{14mu} W}} \\{= {1.08\mspace{14mu} {{µH}.}}}\end{matrix}$

L_(s) can then be calculated (derived from equations 2 & 3):

$\begin{matrix}{L_{s} = \frac{L_{m}}{\frac{f_{\max}^{2}}{f_{\min}^{2}} - 1}} \\{= \frac{1.08\mspace{14mu} {µH}}{\frac{500\mspace{14mu} {kHz}}{377\mspace{14mu} {kHz}} - 1}} \\{= {1.43\mspace{14mu} {µH}}}\end{matrix}$

Following from this C_(s) can be calculated (from equation 3):

$\begin{matrix}{C_{s} = \frac{1}{{L_{s}\left( {2\; \pi \; f_{\max}} \right)}^{2}}} \\{= \frac{1}{1.43\mspace{14mu} {uH}\mspace{14mu} \left( {2\; \pi \; 500\mspace{14mu} {kHz}} \right)^{2}}} \\{= {71\mspace{14mu} {nF}}}\end{matrix}$

To maintain regulation, the circuit is run above resonance so actualvalues of C_(s) will be typically 20% higher to bring the operatingpoint back down (if below resonance was chosen C_(s) would have to bereduced).

As previously mentioned, the efficiency is maximised when R_(load) _(_)_(ref) is equal to the magnetising reactance at the critical frequency(equation 5). It is desirable, therefore, to operate about the middle ofthe constant power range (shown in FIG. 5b ). R_(load) _(_) _(upper) isthe load resistance at which constant power changes to constant voltage.Similarly, R_(load) _(_) _(lower) is the load resistance at whichconstant power changes to constant current.

$\begin{matrix}{R_{load\_ upper} = \frac{V_{load}^{2}}{P}} \\{= \frac{85\mspace{14mu} V_{rms}^{2}}{45\mspace{14mu} W}} \\{= {161\mspace{14mu} \Omega}}\end{matrix}$ $\begin{matrix}{R_{load\_ lower} = \frac{P}{I_{load}^{2}}} \\{= \frac{45\mspace{14mu} W}{1.4\mspace{14mu} A^{2}}} \\{= {23\mspace{14mu} \Omega}}\end{matrix}$

Take the geometric mean of these load resistances to find R_(load) _(_)_(c) (centre or critical load resistance)

$\begin{matrix}{R_{load\_ c} = \sqrt{R_{load\_ upper}R_{load\_ lower}}} \\{= {60\mspace{14mu} \Omega}}\end{matrix}$

As discussed, for maximum efficiency, R_(load) _(_) _(ref) should matchthe impedance of the primary-referred magnetising reactance at f_(c).Hence R_(load) should equal the secondary-referred magnetisingreactance. L_(sec) can therefore be calculated as follows:

$\begin{matrix}{L_{\sec} = \frac{R_{load\_ c}}{2\; \pi \; f_{c}}} \\{= \frac{60}{2\; \pi \; 430\mspace{14mu} {kHz}}} \\{= {22.2\mspace{14mu} {µH}}}\end{matrix}$

Finally the transformer turns ratio can be calculated:

$\begin{matrix}{N = \sqrt{\frac{L_{\sec}}{L_{m}}}} \\{= \sqrt{\frac{22.2\mspace{14mu} {µH}}{1.08\mspace{14mu} {µH}}}} \\{= 4.5}\end{matrix}$

For any particular design it may be necessary to adjust the values dueto the following reasons:

-   -   to maximise efficiency    -   compensate non ideal effect of components (e.g. series        resistance, parasitic capacitance & inductance, non ideal        transformer characteristics such as leakage inductance)    -   make the design practical (e.g. use standard values of        capacitors and a whole number of turns    -   allow margin to meet the requirements due to component        tolerances, temperature etc

In this specific embodiment, the component values were optimised to:

Cs=82 nF Lm=1.1 uH Ls=1.4 uH

N=5 which gives Lsec=24 uH

The following subsections briefly describe how these component valueswere physically implemented.

Capacitor Selection

A low loss capacitor is desired to minimise losses and to ensure thecomponent doesn't get too hot. Ceramic capacitors are ideal and thedielectric type of COG/NPO were used in this embodiment. The capacitorvoltage rating is also important as it shouldn't be exceeded under allload conditions. Ten 250V 8.2 nF 1206 COG/NPO ceramics capacitors inparallel were used in this embodiment.

Inductor and Transformer

In this embodiment, Ferroxcube 3F3 E3216/20 e-core/plate combination wasused as a ferrite core. Ferroxcube 3F3 is supplied by Ferroxcube, asubsidiary of Yageo Corporation, Taiwan. It is a high frequency ferritematerial optimised for frequencies between 200 kHz and 500 kHz. By usingthis material the core losses are minimised. Core losses increasestrongly with increasing flux density. In an inductor, for a particularrequired energy storage, the flux density increases with decreasing airgap (the air gap is the separation between the e-core & plate).Therefore the air gap and the number of turns can be increased todecrease core losses but this has to be balanced with the actualinductance value required and increased resistive losses introduced withthe longer wire/track length.

The same issues apply to the transformer except core losses are due tothe output voltage and the number of turns. Since the output voltage isfixed the number of turns is the only variable that can be changed butagain this has to be balanced with resistive losses. Once the number ofturns is set the air gap can then be adjusted to set Lm. Whatever coreis used, it is best practise to fill the winding space with as muchcopper as possible to minimise resistive losses. In the transformer thevolume of windings is preferably about the same in the primary andsecondary to balance the losses.

The resistive losses can usually be easily calculated but since thecircuit is operating at about 400 kHz skin depth becomes an issue. Theskin depth in copper at 400 kHz is only about 0.1 mm so a solidconductor thicker than this doesn't result in all the copper being used.Litz wire (stranded insulated copper wire twisted together where eachstrand is thinner than the skin depth) can be used to reduce thiseffect. In this embodiment 2 oz PCB tracks (about 0.07 mm thick coppertracks) were used for the windings of both the inductor (L_(s)) and thetransformer to avoid having to wind custom components. The inductor hadtwo turns with an air gap of 0.5 mm between the e-core and plate. Thetransformer had one turn on the primary and five turns on the secondarywith an air gap between the e-core and plate of 0.1 mm.

Modifications and Alternatives

A medical cauterisation device has been described above. As thoseskilled in the art will appreciate, various modifications can be madeand some of these will now be described. Other modifications will beapparent to those skilled in the art.

In the above embodiment, various operating frequencies, currents,voltages etc were described. As those skilled in the art willappreciate, the exact currents, voltages, frequencies, capacitor values,inductor values etc. can all be varied depending on the application andthe values described above should not be considered as limiting in anyway. However, in general terms, the circuit described above has beendesigned to provide an RF drive signal to a medical device, where thedelivered power is desired to be at least 10 W and preferably between 10W and 200 W; the delivered voltage is desired to be at least 20 V_(rms)and preferably between 30 V_(rms) and 120 V_(rms); the delivered currentis designed to be at least 0.5 A_(rms) and preferably between 1 A_(rms)and 2 A_(rms); and the drive frequency is at least 50 kHz.

In the above embodiment, the resonant circuit 28 was formed fromcapacitor-inductor-inductor elements. As those skilled in the art willappreciate, the resonant circuit 28 can be formed from various circuitdesigns. FIG. 9 illustrates another resonant circuit design that can beused in other embodiments. In the design shown in FIG. 9, the resonantcircuit 28 is formed from capacitor-inductor-capacitor elements, withthe load being connected across the second capacitor 78. As shown, inthis design, there is no transformer and so there is no step-up involtage. However, the operation of this embodiment would still be thesame as in the embodiment described above and so a further descriptionshall be omitted. Other resonant circuit designs with multiplecapacitors and inductors in various series and parallel configurationsor simpler LC resonant circuits may also be used.

FIG. 1 illustrates one way in which the batteries and the controlelectronics can be mounted within the handle of the medical device. Asthose skilled in the art will appreciate, the form factor of the handlemay take many different designs.

In the above embodiment, an exemplary control algorithm for performingthe cauterisation of the vessel or tissue gripped by the forceps wasdescribed. As those skilled in the art will appreciate, variousdifferent procedures may be used and the reader is referred to theliterature describing the operation of cauterisation devices for furtherdetails.

In the above embodiment, the RF drive signal generated by the drivecircuitry was directly applied to the two forceps jaws of the medicaldevice. In an alternative embodiment, the drive signal may be applied toone jaw, with the return or ground plane being provided through aseparate connection on the tissue or vessel to be cauterised.

In the above embodiments, the forceps jaws were used as the electrodesof the medical device. In an alternative device, the electrodes may beprovided separately from the jaws.

In the above embodiments, two FET switches were used to convert the DCvoltage provided by the batteries into an alternating signal at thedesired RF frequency. As those skilled in the art will appreciate, it isnot necessary to use two switches—one switch may be used instead ormultiple switches may be used connected, for example, in a bridgeconfiguration. Additionally, although FET switches were used, otherswitching devices, such as bipolar switches may be used instead.However, MOSFETs are preferred due to their superior performance interms of low losses when operating at the above described frequenciesand current levels.

In the above embodiment, the resonant circuit 28 acted as a frequencydependent attenuator. The resonant circuit was designed as asubstantially lossless attenuator, but this is not essential. Theresonant circuit may include lossy components as well, although theresulting circuit will of course be less efficient.

In the above embodiment, the I & Q sampling circuitry 51 sampled thesensed voltage/current signal once every period and combined samplesfrom adjacent periods. As those skilled in the art will appreciate, thisis not essential. Because of the synchronous nature of the sampling,samples may be taken more than once per period or once every n^(th)period if desired. The sampling rate used in the above embodiment waschosen to maximise the rate at which measurements were made available tothe medical device control module 57 as this allows for better controlof the applied power during the cauterisation process.

In the above embodiment, a 24V DC supply was provided. In otherembodiments, lower DC voltage sources may be provided. In this case, alarger transformer turns ratio may be provided to increase the loadvoltage to a desired level or lower operating voltages may be used.

In the above embodiment, a synchronous sampling technique was used toobtain measurements of the load voltage and load current. As thoseskilled in the art will appreciate, this is not essential and other moreconventional sampling techniques can be used instead.

In the above embodiment, the medical device was arranged to deliver adesired power to the electrodes of the end effector. In an alternativeembodiment, the device may be arranged to deliver a desired current orvoltage level to the electrodes of the end effector.

In the above embodiment the battery is shown integral to the medicaldevice. In an alternative embodiment the battery may be packaged so asto clip on a belt on the surgeon or simply be placed on the Mayo stand.In this embodiment a relatively small two conductor cable would connectthe battery pack to the medical device.

1-19. (canceled)
 20. A method of operating a medical device comprisinggenerating an RF signal and applying the RF signal to at least oneelectrode of an end effector of the medical device and controlling afrequency of the generated RF signal to control at least one of a power,a current, and a voltage applied to the at least one electrode.
 21. Amethod of cauterizing a vessel or tissue, the method comprising:gripping the vessel or tissue with an end effector of a medical device;applying an RF signal to at least one electrode of the end effector thatis in contact with the vessel or tissue; and controlling a frequency ofthe RF signal to control at least one of a power, a current, and avoltage applied to the tissue to perform the cauterization.
 22. Themethod according to claim 21, which uses a medical device comprising: ahandle for gripping by a user, an end effector coupled to the handle andhaving at least one electrical contact; a radio frequency, RF,generation circuit coupled to the handle and operable to generate an RFdrive signal and to provide the RF drive signal to the at least oneelectrical contact; wherein the RF generation circuit comprises aresonant circuit.
 23. The method according to claim 21, whereincontrolling the frequency comprises: varying the frequency of the RFsignal to control the power applied to the tissue or vessel; obtainingmeasurements of the impedance of the tissue or vessel; and varying thedesired power applied to the tissue or vessel in dependence upon theobtained impedance measurements.